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Assuming that you could fine a wider and a narrower tire with the EXACT same weight, the EXACT same suppleness, and then pumped them up to the EXACT same pressure, the wider tire would offer less rolling resistance. Unfortunately, this is never the case. Reality is a *****. The best compromise generally runs between 700x23c and 700x28c depending on the aerodynamics demanded by the application.

All else being constant, fat slicks have lower rolling resistance and narrow slicks have lower air resistance. At some speed, AR becomes more significant than RR. Below this threshold, fat slicks are more efficient, above it, thin slicks rule.

Moment of intertia, and hence weight, only really matters when you're accelerating. When maintaining a constant speed, the weight actually helps, right? But because of the decelerating force of rolling resistence and air resistence, you're applying force to maintain that steady speed and in practice, you are going to be constantly declerating and accelerating by small measures even when you are attempting to maintain a steady speed, right? So I'd have to conclude that weight matters a lot.

In the real world, unless you're talking about the differences between a 700x25 and a 700x28 of the same make, weight is going to be a big factor.

Would I be stating the obvious to say that a wide (let's say, 35) 500g slick that takes 75 psi is going to be much slower than a narrow (let's say, 25) 200g slick that takes 120 psi?

There are so many variable involved that make it almost impossible to say which is better.

A 180lb rider needs X amount of air to supoport his/her weight. A 700X23 @120psi and a 700X35 @80psi may have the same amount of air. Which has less rolling resitence? Who knows....

Factors that come into play sre:

1. size of contact patch (amt of rubber on the road)
2. Tire construction (more t.p.i. the better)
3. Type of material
4. Type of inner tube
5. cross sectional area (any speeds less than 25mph or so and aero isnt much of a factor)

I can easily find a 700 X 32 tire/tube combo that will have lower rolling resistence than a 700/23 combo.

Moment of intertia, and hence weight, only really matters when you're accelerating. When maintaining a constant speed, the weight actually helps, right? But because of the decelerating force of rolling resistence and air resistence, you're applying force to maintain that steady speed and in practice, you are going to be constantly declerating and accelerating by small measures even when you are attempting to maintain a steady speed, right? So I'd have to conclude that weight matters a lot.

I don't know what you consider a lot, but the people who study this (by experiment and analysis) find that total wheel inertial effects make less than 0.01% difference and weight makes less than 0.1% difference. Just listing a bunch of competing effects without an estimate of their magnitude is no basis for conclusions. http://www.biketechreview.com/archive/wheel_theory.htm

There is a reason why road racing is done on very narrow tyres, pumped up to high pressure.
Whether you want that on your daily ride, where comfort and ability to have a reasonably good ride on various surfaces come to play, is another matter.

I don't know what you consider a lot, but the people who study this (by experiment and analysis) find that total wheel inertial effects make less than 0.01% difference and weight makes less than 0.1% difference. Just listing a bunch of competing effects without an estimate of their magnitude is no basis for conclusions. http://www.biketechreview.com/archive/wheel_theory.htm

You know I've read a number of these scientific analyses that basically make the same claims and they seem very convincing. Unfortunately they do not at all match what I feel in the seat of my pants so I'm left wondering just why that is. When I changed my MTB from light weight 2.1-in kevlar bead cross country tires to heavy 2.5-in steel bead tires I got dramatically improved traction, especially on soft surfaces like sand or gravel but I also found that guys who I use to beat to the top of any hill could now drop me at will on any climb. Since aerodynamics would play little roll on a MTB I would have to assume the weight of the tires was the primary cause of this. I noticed a similar if less dramatic effect on my vintage road bike when I changed from light weight tubulars to clinchers. Suddenly the bike seemed sluggish to accelerate or turn. I weighed the wheel sets and tires and found the clincher setup to be about 420-gr heavier per wheel than the tubulars. The clincher were slightly wider but both were running about 100-psi pressure. I'm not yet convinced that all the appropriate factors are being included in the formulas those guys are using.

Unfortunately they do not at all match what I feel in the seat of my pants so I'm left wondering just why that is.

How well calibrated is the seat of your pants? I know for a fact that my red bike feels faster than my blue one. I also know that my blue bike is just as fast as the red one. There is no contradiction there.

i thought skinny tires were faster 'cause they have a smaller contact patch on the road. as well, they're more aero (or should be). why is a fatter tire faster, in that case, then?

when i went from a POS 1.25" tire to a POS 1.125" tire on my road bike, my speed increased!

on my POS FS mountain bike which weighed 40lbs and rode on 2.1" tires, i could average 25km/h on the road. now, on my 28lb fancy pants HT mountain bike with 2.35" tires, i can average about 19km/h on the road. i'm a stronger rider now than i was when i rode the POS FS mtb, too.

i thought skinny tires were faster 'cause they have a smaller contact patch on the road.

You might want to think about that a little more. Contact area is just weight divided by tire pressure. Also, there are a lot more factors at play in rolling resistance for lugged tires compared to smooth or siped ones.

You might want to think about that a little more. Contact area is just weight divided by tire pressure.

but don't the skinnier ones allow you to run higher pressures, thus reducing contact area? i've not had too much experience with higher-end rubber, but among cheap stuff, the thinner the tire, the higher the PSI potential.

my 2.35's max is 50
my 2.1's max is 60
my 1.25's were 90
i saw a set of nicer 1.125's with a 110 max

It's not just that skinniness inherently allows higher pressure, it's more that skinniness requires higher pressure...and the tires are built to withstand it. The size of the contact patch is mainly determined by the tire pressure, although the stiffness of the tire carcass also plays a role. If you have a skinny tire and a fat tire at the same pressure, the skinny tire's contact patch will be longer and narrower, and that of the fat tire will be shorter and wider, but the areas will be about the same.

Tread design and tire material and construction play a big role in determining the amount of drag. Two tires of the same size and running at the same pressure can have quite a bit of difference in drag.

How well calibrated is the seat of your pants? I know for a fact that my red bike feels faster than my blue one. I also know that my blue bike is just as fast as the red one. There is no contradiction there.

Well, when I'm looking at the seat of somebody else's pants who used to be looking at the seat of mine, that's pretty good indication that I'm going slower But of course it is all subjective. That's why I said, the science behind it sounds very convincing but...

It's also quite possible that the difference is small and for such a brief duration (such as when changing direction or accelerating hard) that it makes no practical difference on the road but is still detectable to the rider, resulting in that "slower" feeling.

edit: Just to show how subjective these things can be, I remember way back in the day the "fastest" car I ever drove was my buddy's 1965 Ponitac Grand Prix with a 421 tri-power engine. In truth it was at least a second slower than my or my other friend's heavily modified Z28 and Mustang GT but man was it scary. It was as big as a yacht and made a deafening roar when you cracked open the three two barrels. The softly sprung front end seemed like it wanted to come off the ground. You had to steer for all you were worth with the big manhole size steering wheel and all you could do was nudge the thing in the direction you wanted to go as the rear end fish tailed out from under you. And don't even think about stopping as the big 8-lug aluminum finned drum brakes were all show and no go, er stop. It was Mr. Toad's wild ride. You had white knuckles the whole time. IIRC it actually only turned 14.8 in the 1/4 mile but my God it was a beast. The Z and the Mustang turned 13's but were so well behaved by comparison as to seem tame.

There is a reason why road racing is done on very narrow tyres, pumped up to high pressure.
Whether you want that on your daily ride, where comfort and ability to have a reasonably good ride on various surfaces come to play, is another matter.

Bingo.

A strong rider can go fast on anything. However, weight and aero are important even if deflection is greater on a smaller tire pumped to the same pressure as a wider tire. If wide tires really were faster, don't you suppose people who got paid to race would want them?

One way to test the issue yourself is to get different sets of tires and see what happens. I'm not against wide tires (I have 28's on my commuter), but I definitely want 23's on the racer.

They also mean an increased cross sectional area; air drag is exponential, so this can get pretty noticeable.

Moment of inertia affects acceleration, and air drag affects speed.

I'm not so sure about how fatter slicks get less rolling resistance, but basically the comparison will be a balance between the different phenomena.

more weight does not necessarily mean a higher moment of inertia, which goes like the mass times distance from the axis of rotation squared. If you put mass at the axis of rotation then there is no increase in the moment of inertia

Speed goes like your velocity at bike speeds <75mph and it goes like the velocity squared after that.

As for the 'scientific' article it lacked any isolation of external variables to be of any real use. Was Kraig Willets conditioning the same for each of these trials. This would be an extremely hard thing to control for and I doubt it was. Mixing vectors and scalars and not appropriately signing power losses relative to power gains is also suspect.

Friction has nothing to do with surface area. It is a material property only.

more weight does not necessarily mean a higher moment of inertia, which goes like the mass times distance from the axis of rotation squared. If you put mass at the axis of rotation then there is no increase in the moment of inertia

So all I need is wide tires that concentrate all mass in the center of my hubs -- I'm on this!

Quote:

Originally Posted by jonestr

Friction has nothing to do with surface area. It is a material property only.

Handy information. I intend to cut my brake pads to 5% their current size to save weight. This should actually improve braking performance since there will be less mass to stop.